Move directive initialisation to the boot method

[owenvoke]

As per the conversation in #23 it seems that the Blade::directive() usage in the register() method isn't initialised. In Laravel 5.8, the facadeAccessor() for the Blade facade now returns the accessor string instead of an instance like in previous versions (see laravel/framework#25497).

According to the documentation, using facades in the register method isn't recommended as they may not have been defined yet, so they should be used in the boot method, which occurs after all register methods have finished.

I've tested this in Laravel 5.5, 5.6 and 5.8. Also, @HDVinnie has tested it on dev servers using Laravel 5.7 and 5.8 as per #23 (comment).

The PHPUnit tests seem to work as expected on Travis and locally, but @Elhebert, please feel free to test this.

Fixes #23

[Elhebert]

Nice catch!

Thx for the fix.

[owenvoke]

Cheers! I guess this can be a v1.5.1 whenever. 👍

[Elhebert]

I'm on it as soon as I'm on my laptop.

[Elhebert]

Done ✅

[HDVinnie]

Thanks @Elhebert @pxgamer Great Package! 💯